Micromachined microelectromechanical systems (MEMS) based resonators are receiving continuously increasing attention due to their small sizes compared to conventional crystal-based resonators as well as their potential for integration with other MEMS resonators and circuits on the same chip which can be especially advantageous for handheld electronic device applications (e.g. smartphones and tablets) where weight, size, and cost are particularly critical parameters.
Micromachined resonators can be operated through two main widespread actuation mechanisms: piezoelectric or electrostatic, each method having its specific advantages and drawbacks. Piezoelectric devices such as surface acoustic wave (SAW) and film bulk acoustic resonators (FBAR) are already widely used in timing applications. Piezoelectric FBARs generally exhibit high electromechanical transduction efficiencies and low signal transmission losses, resulting in low motional resistances, which is very advantageous as it simplifies the design constraints of the associated electronic circuitry and results in lower power consumption. Further, piezoelectric devices do not require any DC voltage for operation.
However, conventional piezoelectric FBAR devices generally suffer from lower quality factors, notably limited by the resonator-to-electrode strain loss at the interface and their resonance frequencies are limited by the piezoelectric layer thickness, as they utilize out-of-plane vibration modes. This limitation makes it impractical to achieve more than one resonant frequency on the same chip. Lateral mode devices in contrast use in-plane modes, and can achieve different resonant frequencies by adjusting the device dimensions. Rinaldi et al. in “High Frequency AlN MEMS Resonators with Integrated Nano Hot Plate for Temperature Controlled Operation” (IEEE Proc. Int. Freq. Control Symp., pp. 1-5, 2012) employed integrated nanoscale heating elements for temperature control of aluminum nitride MEMS contour mode resonators. The heating elements are separated from the resonators structures by sub-micron air gaps in order to maintain the electromechanical properties of the devices.
In contrast, Hummel et al. in “Highly Reconfigurable Aluminum Nitride MEMS Resonator using 12 Monolithically Integrated Phase Change Material Switches” (IEEE Proc. Int. Conf. Solid State Sens. Actuators, Microsyst., pp. 323-326, 2015), integrated switches serve to create a reconfigurable contour mode resonator. In contrast, Gong et al. in “Design and Analysis of Lithium Niobate Based High Electromechanical Coupling RF-MEMS Resonators for Wideband Faltering” (IEEE Trans. Micro. Theory Techn., Vol. 61, No. 1, pp. 403-414, 2013), exploit another type of lateral vibrating piezoelectric resonator is presented based on lithium niobate. It is also important to note that piezoelectric actuation does not provide inherent means for frequency tuning.
In contrast electrostatic resonators, which rely mostly on vibratory resonance, operate either in a flexural or bulk mode. Bulk-mode devices typically exhibit high stiffness, and are consequently less prone to thermoelastic damping, compared to flexural devices, allowing them to achieve large quality factors (>10,000), even at atmospheric pressure. However, electrostatic actuation causes these resonators to exhibit lower electromechanical transduction efficiency and significant signal transmission loss, leading to higher motional resistance than typical piezoelectric devices. This can however be mitigated by increasing the applied DC polarization voltage and utilizing sophisticated technologies to realize very thin lateral transduction gaps (<100 nm). Electrostatic resonators can also benefit from the electrostatic spring softening phenomenon to allow for tuning the resonance frequency by varying the polarization voltage. This effect cannot be replicated for typical piezoelectric devices. Within the prior art capacitive bulk-mode disk resonators exhibiting quality factors of 2,000-150,000 and motional resistances of 1.5 kΩ-1MΩ using polarization voltages<20 V have been demonstrated. Further, within the prior art, see for example Hung et al. in “Capacitive-Piezoelectric AlN Resonators with Q>12,000” (IEEE Proc. Int. Conf. Micro Electro Mech. Syst., pp. 173-176, 2011), actuation and sensing of a bulk-mode disk resonator fabricated from aluminum nitride (AlN) has been reported exploiting a combination of piezoelectric and capacitive methods. The reported device resonates at 51 MHz with quality factor of approximately 13,000 and a loss of approximately −34 dB. However, no tuning capability is reported. In contrast, Schneider et al. in “On/off Switchable High-Q Capacitive-Piezoelectric AlN Resonators” (IEEE Proc. Int. Conf. Micro Electro Mech. Syst., pp. 1265-1268, 2014), a radial mode AlN resonator is presented which employs a capacitive-piezoelectric actuation scheme similar to Hung, using metal electrodes above and beneath the disk structure. The electrostatic electrodes beneath the disk structure are also used to pull it down to the substrate and consequently power the resonator off upon the application of a switching DC voltage.
Notably, all the aforementioned prior art bulk-mode designs require complex fabrication process steps to realize submicron gaps in order to avoid excessive motional resistances. In Serrano et al. “Electrostatically Tunable Piezoelectric-on-Silicon Micromechanical Resonator for Real-Time Clock” (IEEE Trans. Ultrason., Ferroelectr., Freq. Control, vol. 59, no. 3, pp. 358-365, 2012) a flexural (out-of-plane) piezoelectric resonator is presented. The device employs a metal electrode beneath the suspended structure for electrostatic tuning. However, it operates at a relatively low frequency of approximately 32 kHz due to the low stiffness flexural mode of operation. In Hodjat-Shamami et al. in “A Dynamically Mode-Matched Piezoelectrically Transduced High Frequency Flexural Disk Gyroscope” (IEEE Proc. Int. Conf. Micro Electro Mech. Syst. (MEMS), 2015, pp. 789-792) a different method of frequency tuning is presented, where electromechanical feedback of the drive-mode displacement signal is used in order to achieve dynamic mode-matching of the proposed piezoelectric gyroscope. Electrostatic resonating bulk structures using commercial micromachining technologies have been fabricated within the prior art but the devices had to operate at high voltages (>50 V) in order to overcome the dimension limitations on the transducer gap imposed by the technology.
Within the prior art of Elsayed et al. in “A Combined Comb/Bulk Mode Gyroscope Structure for Enhanced Sensitivity” (IEEE Proc. Int. Conf. Micro Electro Mech. Syst. (MEMS), 2013, pp. 649-652) and “A Novel Comb Architecture for Enhancing the Sensitivity of Bulk Mode Gyroscopes” (Sensors, vol. 13, pp. 16641-16656, 2013) combs were added to the bulk-resonating structure in order to improve overall sensitivity and allow for operation at lower voltage, at the expense of increased area.
Accordingly, it would be beneficial to provide MEMS based bulk mode resonators that overcome the aforementioned limitations of prior art designs. It would be beneficial to provide MEMS designers with bulk mode resonator designs that combine transverse piezoelectric actuation with bulk mode resonance in order to achieve low motional resistance without requiring any DC voltage, reasonable quality factor (even in air), and a relatively high resonance frequency. It would be further beneficial to provide MEMS circuit designers with a bulk mode resonator design that reduces fabrication complexity by employing a single metal electrode layer topping the resonator structure rather than conventional FBAR designs within the prior art that require additional metal electrodes beneath the structure. Beneficially, such designs also lower the Q-limiting resonator-to-electrode strain loss compared to these conventional FBAR designs.
It would be further of benefit to MEMS circuit designer to provide such MEMS bulk mode resonators without requiring narrow transduction gaps allowing their fabrication on relatively low complexity and low resolution commercial MEMS processes such as the MEMSCAP PiezoMUMPs process, for example. Further, beneficially embodiments of the invention provide MEMS circuits with electrostatic tuning and provide for designs that combine the advantages of piezoelectric actuation and bulk-mode resonators.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.